Where exactly do we see a phaserproof barrel?
We have seen fancy things such as a phaserproof painting
(Damn, no blu-ray screencaps at TrekCore!)
...but no proof that it was hit with a kill beam. Stun beams are a good combat option in most situations, as it rarely is tactically necessary to permanently remove an enemy combatant from the equation. Kill settings are useful mainly when you want to assassinate, or when you want to wrestle permanent control of a location out of the hands of an enemy that otherwise might have the ability to regroup. But our heroes and even our villains fairly seldom assassinate, as their true tactical goals lie elsewhere. And most fights in Star Trek do not involve wrestling control of a location in the classic infantry manner - rather, they are exchanges of fire in a starship corridor or on a planetside location in a short-lived ambush or raid situation that is not going to have any permanent consequences.
So, is there a situation where a barrel or comparable structure would have withstood a kill phaser?
There are situations where a definite kill phaser hits a wall and/or a floor in, say, "In the Hands of the Prophets".
Sometimes TPTB introduce a scorch mark there, sometimes not; typically, it disappears within a few shots anyway. But walls have a good excuse to be phaserproof.